The invention relates generally to a process for the adsorptive separation of low molecular weight hydrocarbons. In a preferred embodiment, the invention relates to the kinetic separation of propylene from other hydrocarbons, in particular, from propane. The process employs at least one substantially acid-free zeolite having an 8-member ring channel system. Preferred zeolite structure types are CHA and ITE.
It has long been known that certain porous substances such as zeolites which have certain selective adsorption characteristics are useful in separating a hydrocarbon mixture into its component parts. Similarly, it is well known in the separation art that certain crystalline zeolites can be used to separate certain hydrocarbons from feed mixtures.
The separation of propylene-propane mixtures is one of the most important operations in the petrochemical industry. Such mixtures usually result from the thermal or catalytic cracking of hydrocarbons, and the majority of them represent coproducts with ethylene. Their separation is of great economic consequence, since the separated propylene has many uses, one of the most important being as monomer feedstock for polypropylene elastomer production. For most end uses the propylene must have a high purity. The propane fraction can be recycled to the cracking step or used separately, e.g., as liquefied petroleum gas (LPG) for heating.
The conventional method for separating the propylene-propane mixture is fractional distillation. The relative volatility for the mixture is in the range of 1.09-1.15 (Laurance and Swift, 1972), depending on composition and pressure of operation. A large number of contacting stages are required (over 100), and the associated high reflux ratio requires a large input of energy. The U.S. Department of Energy has reported that the propylene/propane separation is the most energy-intensive single distillation practiced commercially (Wiley, 1992).
A significant amount of the light olefins produced during the refining of crude oil is used as refinery fuel. Cost-effective separation technologies will be required that can withstand the harsh operating environment of a petroleum refinery. In addition to traditional refinery and olefin plant applications, paraffin dehydrogenation units for the production of olefins from natural gas liquids (NGL) have recently been brought on stream. For these small units, typically less than 5,000 bbl/day of olefin production, novel separation technologies may be more competitive with more traditional unit operations.
Recent federal regulations, such as the Clean Air Act, mandate that hydrocarbon emissions from refineries and chemical plants be reduced to low levels. To reduce the economic penalty of environmental compliance, low-cost hydrocarbon separation technologies are required. For facilities located in nonattainment areas, the need to reduce hydrocarbon losses is especially critical. Facilities can no longer afford to dispose of waste hydrocarbon streams in their flare systems. Purge streams from polyolefin reactors and vents from polymer storage facilities, which were once flared, must be redirected to recovery systems. For streams containing a mixture of paraffinic and olefinic material, economic and process considerations may dictate that the olefin be recovered and recycled.
Propylene is obtained by recovery from petroleum feed stocks which include mixtures of olefins and paraffins. Although propylene can be separated without difficulty from certain olefins and paraffins by fractional distillation, that separation technique is less useful for olefins and paraffins having similar boiling points and similar volatilities, in particular propylene and propane. For this reason, the separation of propylene in highly purified form from a mixture containing propylene and propane is difficult and, as noted above, consumes a large amount of energy.
It is an object of this invention to provide a zeolite adsorbent material that will separate propylene from propylene/propane and propylene/olefin mixtures in a cost efficient manner. To that end, the zeolites used in practicing this invention are relatively inexpensive. The zeolites also have a high adsorption capacity for propylene and a high degree of selectivity for propylene over propane and other olefins. These zeolites are, therefore, suitable for large scale commercial operations.
It is another object of this invention to provide a more environmentally acceptable process for obtaining highly purified propylene from a propylene/propane mixture.
It is yet another object of the invention to carry out a separation of other light hydrocarbons using the kinetic based process of this invention in an energy-efficient manner. It is a further objective of the invention to provide a separation process that is highly efficient in having high diffusion rates for propylene relative to propane.
The invention can be summarized as encompassing certain zeolite adsorbents, methods for preparation of such zeolites, and their use in methods of separating a mixture containing at least two hydrocarbons.
The zeolites of the invention comprise or consist of zeolites having a channel structure that has 8-member rings of tetrahedra defining diffusion in the channels. The zeolites are also substantially acid-free. Zeolites containing as constituents both aluminum and silicon must have an Si:Al molar ratio of at least about 200. Low silicon zeolites such as aluminophosphate-type zeolites are also suitable, provided they satisfy the other requirements stated herein. Examples of suitable aluminophosphate-type zeolites and zeolites containing as constituents aluminum and silicon are set forth below.
The zeolites of the invention are also characterized as having a propylene/propane diffusion ratio of greater than about 50 (at 80xc2x0 C. and 600 torr hydrocarbon pressure) and having a propylene absorption capacity greater than 40 mg/g at 80xc2x0 C. and 600 torr propylene pressure. Suitable zeolites are those having an 8-member ring channel structure and a molar ratio of silica to aluminum of at least about 200:1 and may have at least one low activity metal cation to minimize the acid activity of the zeolite so that it does not result in significant polymerization of the olefins. Preferred zeolites are those having CHA and ITE-type structures.
The invention also provides a process for separating hydrocarbons from a hydrocarbon mixture and, in particular, propylene from a propane/propylene mixture using the above-described zeolite adsorbent. The process comprises the steps of contacting the hydrocarbon mixture with at least one of the zeolites to effect separation of propylene. In a preferred embodiment, the zeolite adsorbent separates propylene from propylene/propane and propylene/propane/ethylene mixtures.